Even under substantial levels of wind forcing, rain is believed to play an important role in air-sea fluxes. Numerous laboratory measurements have shown that the generation of ring waves by the impact of rain on an air-water interface is substantial, especially in light wind conditions (Bliven et al., 1993, 1997 Harrison et al., 2012 Poon et al., 1992 Yang et al., 1997). Under light rainfall, they may be the radially propagating features known as “ring waves.” A ring wave is characterized by an impact crater, downward and outward flow, and a rapid upward jet (Morton et al., 2000). Under heavy rainfall, these may be the chaotic pockmarks, which result from the frequent and violent interaction between raindrops and impact craters. However, rain also generates centimeter-scale water surface undulations of its own. This fact is often emphasized as the key feature of the relationship between rain and ocean surface waves (Cavaleri & Bertotti, 2017 Cavaleri et al., 2015 Tsimplis & Thorpe, 1989). Rainfall has long been known to “knock down the sea”-that is-to attenuate ocean surface gravity waves (Reynolds, 1900 Tsimplis & Thorpe, 1989). Precipitation also has a profound impact on the ocean surface wave state. This fresh lens is typically tens of centimeters thick, with turbulent kinetic energy dissipation falling orders of magnitude over the upper half meter of the water column (Zappa et al., 2009). This phenomenon has been extensively studied and documented in the literature-particularly in recent years (e.g., Doeschate et al., 2019 Dong et al., 2017 Drushka et al., 2016 Volkov et al., 2019 Shcherbina et al., 2019 Thompson et al., 2019). Under low levels of wind forcing, this lens-often several meters thick-forms what is known as a “slippery layer” (Anderson et al., 1996 Kudryavtsev & Soloviev, 1990), accelerating past the saline water at the base of the lens (Shcherbina et al., 2019 Wijesekera et al., 1999). ![]() ![]() It is understood that rainfall on the ocean creates a thin, highly turbulent fresh lens near the surface (Harrison & Veron, 2017 Peirson et al., 2013 Zappa et al., 2009), which enhances air-sea flux but temporarily stalls vertical mixing (Ho et al., 1997 Schlüssel et al., 1997 Zappa et al., 2009). In areas which typically see low levels of wind forcing and high levels of rainfall, such as the Western Equatorial Pacific, rain has been observed to have a substantial impact on air-sea fluxes (Turk et al., 2010). Rain has been shown to alter major circulation patterns (Lee et al., 2019) and stir the upper ocean (Abe et al., 2019) through the large-scale freshening of the ocean's near-surface layer. High levels of rainfall are commonly observed over the tropical regions of Earth's oceans, impacting physical processes that influence weather and climate from the microscale to the basin scale.
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